Thermostable omega-transaminases

ABSTRACT

Thermostable omega-transaminases, particularly thermostable omega-transaminases which have a high reaction rate and which are tolerant to high concentrations of donor amine, can be used to enrich enantiomerically a mixture of chiral amines or to synthesize stereoselectively one of a pair of chiral amines in which the amino group is bound to a non-terminal, chirally substituted, carbon atom.

This application is a division of Ser. No. 11/295,696 filed Dec. 7, 2005, now matured into U.S. Pat. No. 7,172,885, which claims priority to provisional application Ser. No. 60/634,526 filed Dec. 10, 2004. Each of these applications is incorporated by reference in its entirety herein.

FIELD OF THE INVENTION

The invention relates to omega-transaminases and their use in enantiomeric enrichment and stereoselective synthesis.

BACKGROUND OF THE INVENTION

Chiral molecules, such as single-enantiomer drugs, can be produced by biotransformation, asymmetric synthesis using chiral catalysts or reagents, resolution or separation techniques, and the like. There is a continuing need in the art for improved and efficient methods of preparing chiral molecules.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Typical reaction curves to follow the formation of product amine using of 5 g/L and 2.5 g/L of the enzyme CNB03-03 (SEQ ID NO:12) for stereoselective synthesis at 58–60° C.

FIG. 2. Graph showing initial rates of activity of mutants CNB05-01 (SEQ ID NO:16) and CNB05-02 (SEQ ID NO:18).

FIGS. 3A–B. Graphs showing relative enzyme activities of CNB05-01 (SEQ ID NO:16) and CNB03-03 (SEQ ID NO:12). FIG. 3A, values of initial rates. FIG. 3B, rates over time.

DETAILED DESCRIPTION OF THE INVENTION

Transaminases, also called aminotransaminases, catalyze the transfer of an amino group from a donor amine to an amine acceptor molecule. Omega-transaminases (co-transaminases) transfer amine groups which are separated from a carboxyl group by at least one methylene insertion. The invention provides thermostable omega-transaminases, particularly thermostable omega-transaminases which have a high reaction rate and which are tolerant to high concentrations of donor amine. Thermostable omega-transaminases of the invention can be used to enrich enantiomerically a mixture of chiral amines or to synthesize stereoselectively one of a pair of chiral amines in which the amino group is bound to a non-terminal, chirally substituted, carbon atom. Methods of the invention provide cost-effective and specific means for producing chiral molecules.

Thermostable Omega-Transaminases

The amino acid sequence of a wild-type omega-transaminase from Arthrobacter citreus is shown in SEQ ID NO:2. Thermostable omega transaminases of the invention comprise an amino acid sequence which differs from the amino acid sequence shown in SEQ ID NO:2 at each of positions 242, 245, 255, and 268 with one or more additional amino acid substitutions at positions 46, 48, 60, 164, 185, 186, 195, 197, 205, 252, 323, 409, 424, and 436.

For example, some thermostable transaminases have an additional amino acid substitution at position 46 (e.g., SEQ ID NOS:12, 14, 16, and 18) or at position 185 (e.g., SEQ ID NOS:16 and 18). Others have additional amino acid substitutions at positions 48, 195, and 197 (e.g., SEQ ID NOS:8, 10, 12, 14, 16, and 18); positions 46, 48, 195, and 197 (e.g., SEQ ID NOS:16 and 18); positions 60, 164, 186, 252, 409, and 426 (e.g., SEQ ID NOS:6, 8, 10, 12, 16, and 18); positions 48, 60, 164, 186, 195, 197, 252, 409, and 436 (e.g., SEQ ID NOS:8 and 12); positions 48, 60, 164, 186, 195, 197, 252, 409, 424, and 436 (SEQ ID NOS: 8, 10, and 12); or positions 46, 48, 60, 164, 185, 186, 195, 197, 409, 424, and 436 (e.g., SEQ ID NOS:16 and 18).

Certain thermostable omega transaminases of the invention comprise an amino acid sequence which differs from the amino acid sequence shown in SEQ ID NO:2 at each of positions 60, 164, 186, 242, 245, 252, 255, 268, 409, and 436; in these transaminases, one or more substitutions also can be made at positions 46, 48, 195, 197, and 424.

Other thermostable omega transaminases of the invention comprise an amino acid sequence which differs from the amino acid sequence shown in SEQ ID NO:2 at each of positions 46, 48, 195, 197, 242, 245, and 255; in these transaminases, one or more substitutions also can be made at positions 60, 164, 185, 186, 205, 252, 323, 409, 424, and 436.

Preferred substitutions are provided in Table 1 and include all possible combinations of the possible substitutions shown. The most preferred substitutions are indicated in bold and underlined.

TABLE 1 Preferred amino acid substitutions with respect to SEQ ID NO:2. amino acid position no. wild-type thermostable omega-transaminase 46 Met Gly, Ser, Thr , Cys, Tyr, Asp, Glu, Ala, Val, Leu, Ile, Pro, Phe, and Trp 48 Asp Gly , Ser, Thr, Cys, Tyr, and Glu 60 Tyr Gly, Ser, Thr, Cys , Asp, and Glu 164 Tyr Ala, Val, Leu, Ile, Pro, Phe , Trp, and Met 185 Tyr Gly, Ser, Thr, Cys , Asp, and Glu 186 Asp Gly, Ser , Thr, Cys, Tyr, and Glu 195 Pro Gly, Ser , Thr, Cys, Tyr, Asp, Glu, Ala, Val, Leu, Ile, Phe, Trp, and Met 197 Met Gly, Ser, Thr , Cys, Tyr, Asp, Glu, Ala, Val, Leu, Ile, Pro, Phe, and Trp 205 Cys Tyr , Gly, Ser, Thr, Asp, and Glu 242 Ala Val , Leu, Ile, Pro, Phe, Trp, and Met 245 Ala Gly, Ser, Thr , Cys, Tyr, Asp, and Glu 252 Ile Ala, Val , Leu, Pro, Phe, Trp, and Met 255 Phe Ala, Val, Leu, Ile , Pro, Trp, and Met 268 Asp Gly, Ser , Thr, Cys, Tyr, and Glu 323 His Tyr , Gly, Ser, Thr, Cys, Asp, and Glu 409 Thr Lys, Arg , and His 424 Lys Glu , Asp, Arg, and His 436 Val Ala , Leu, Ile, Pro, Phe, Trp, and Met

Omega-transaminases with the substitutions described in Table 1 are thermostable. “Thermostable” means that the enzyme is active (able to carry out the reaction, either stereoselective synthesis or enantiomeric enrichment, as indicated by the continued formation of product) and stable (active for at least 10 hours) up to at least 40° C., preferably up to at least 50–60° C. (e.g., at least 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, or 60° C.), even more preferably up to at least 55–63° C. (e.g., at least 55, 56, 57, 58, 59, 60, 61, 62, or 63° C.). Activity is measured by the monitoring the formation of product as a function of time in a reaction. Typical stereoselective synthesis and enantiomeric enrichment reactions are described in Example 1.

Such thermostable omega-transaminases also have a greater tolerance to high donor amine concentrations than the wild-type enzyme (i.e., they are active and stable) in the presence of high concentration of amine. Preferred thermostable omega-transaminases are active and stable for at least 10 hours in the presence of >500 mM amine. Omega-transaminases with optional additional substitutions at positions 46, 48, 195, 197, and 424 also have higher reaction rates than the wild-type enzyme. These higher reaction rates typically are 1.2 to 3 times higher than the reaction rate of the wild-type enzyme as measured by the rate of formation of product (see Example 1).

The amino acid sequences of several thermostable omega-transaminases are shown in SEQ ID NOS:6 (“Chir9867”), 8 (“CNB03-01”), 10 (“CNB03-02”), 12 (“CNB03-03”), 14 (“CNB04-01”), 16 (“CNB05-01”), and 18 (“CNB05-02”) and in Table 2, below. In Table 2, amino acid substitutions with respect to the wild-type sequence are shown in bold. Thermostable omega-transaminases of the invention do not have the amino acid sequence shown in SEQ ID NO:4 and encoded by the nucleotide sequence shown in SEQ ID NO:3; SEQ ID NO:4 is the amino acid sequence of an omega-transaminase which is tolerant to high donor amine concentrations but which is not thermostable.

TABLE 2 Amino acid substitutions in thermostable omega-transaminases. CNB04-01 CNB05-01 CNB05-02 amino acid wild-type Chir9867 CNB03-01 CNB03-02 CNB03-03 (SEQ ID (SEQ ID (SEQ ID position (SEQ ID NO:2) (SEQ ID NO:6) (SEQ ID NO:8) (SEQ ID NO:10) (SEQ ID NO:12) NO:14) NO:16) NO:18) 46 Met Met Met Met Thr Thr Thr Thr 48 Asp Asp Gly Gly Gly Gly Gly Gly 60 Tyr Cys Cys Cys Cys Tyr Cys Cys 164 Tyr Phe Phe Phe Phe Tyr Phe Phe 185 Tyr Tyr Tyr Tyr Tyr Tyr Cys Cys 186 Asp Ser Ser Ser Ser Asp Ser Ser 195 Pro Pro Ser Ser Ser Ser Ser Ser 197 Met Met Thr Thr Thr Thr Thr Thr 205 Cys Cys Cys Cys Cys Cys Tyr Cys 242 Ala Val Val Val Val Val Val Val 245 Ala Thr Thr Thr Thr Thr Thr Thr 252 Ile Val Val Val Val Val Val Ile 255 Phe Ile Ile Ile Ile Ile Ile Ile 268 Asp Ser Ser Ser Ser Ser Ser Ser 323 His His His His His His His Tyr 409 Thr Arg Arg Arg Arg Thr Arg Arg 424 Lys Glu Glu Lys Glu Lys Glu Glu 436 Val Ala Ala Ala Ala Val Ala Ala

Preparation of Thermostable Omega-Transaminases

Thermostable omega-transaminases of the invention can be produced by any suitable means known in the art (e.g., recombinantly, for example in a host cell, or by chemical synthesis). Preferably the enzymes are produced using high cell density fermentation with recombinant E. coli.

Recombinant Production of Thermostable Omega-Transaminases

Nucleic Acid Molecules

The sequence listing provides coding sequences for preferred thermostable omega-transaminases of the invention (SEQ ID NOS:5, 7, 9, 11, 13, 15, and 17 encode SEQ ID NOS:6, 8, 10, 12, 14, 16, and 18, respectively). Any nucleotide sequence which encodes a particular thermostable omega-transaminase, however, can be used to produce that enzyme recombinantly. For example, sequences encoding a thermostable omega-transaminase can be synthesized, in whole or in part, using chemical methods well known in the art (see Caruthers et al., Nucl. Acids Res. Symp. Ser. 215–223, 1980; Horn et al. Nucl. Acids Res. Symp. Ser. 225–232, 1980). Alternatively, nucleic acid molecules encoding the wild-type omega-transaminase of SEQ ID NO:2 (e.g., SEQ ID NO:1) can be modified to encode a thermostable omega-transaminase of the invention. If desired, such molecules can be isolated from Arthrobacter citreus bacteria using standard nucleic acid purification techniques or can be synthesized using an amplification technique, such as the polymerase chain reaction (PCR), or by using an automatic synthesizer. Methods for isolating nucleic acids are routine and are known in the art. Any such technique for obtaining a nucleic acid molecules can be used to obtain a nucleic acid molecule which encodes the wild-type omega-transaminase.

cDNA molecules encoding thermostable omega-transaminases of the invention can be made with standard molecular biology techniques, using mRNA as a template. cDNA molecules can thereafter be replicated using molecular biology techniques well known in the art. An amplification technique, such as PCR, can be used to obtain additional copies of polynucleotides of the invention.

If desired, the nucleotide sequences disclosed herein can be engineered using methods generally known in the art to alter coding sequences for thermostable omega-transaminases of the invention for a variety of reasons, including but not limited to, alterations which modify the cloning, processing, and/or expression of the polypeptide or mRNA product. Sequence modifications, such as the addition of a purification tag sequence or codon optimization, can be used to facilitate expression. These methods are well known in the art and are further described, for example, in PCT/US04/024868.

Expression Vectors

A nucleic acid molecule which encodes a thermostable omega-transaminase of the invention can be inserted into an expression vector which contains the necessary elements for the transcription and translation of the inserted coding sequence. Methods which are well known to those skilled in the art can be used to construct expression vectors containing sequences encoding enzymes of the invention and appropriate transcriptional and translational control elements. These methods include in vitro recombinant DNA techniques as well as synthetic techniques.

Host Cells

A host cell can be prokaryotic or eukaryotic. Useful host cells include Arthrobacter citreus itself, E. coli, Bacillus subtilis, Vibrio cholerae, Salmonella typhi, Salmonella typhimurium, Neisseria lactamica, Neisseria cinerea, Mycobacteria (e.g., M. tuberculosis), yeasts, etc. Many types of host cells are available, for example, from the American Type Culture Collection (ATCC; 10801 University Boulevard, Manassas, Va. 20110-2209) and can be chosen to ensure the correct modification and processing of a foreign protein. See, e.g., WO 01/98340.

Expression constructs can be introduced into host cells using well-established techniques. Such techniques include, but are not limited to, transferrin-polycation-mediated DNA transfer, transfection with naked or encapsulated nucleic acids, liposome-mediated cellular fusion, intracellular transportation of DNA-coated latex beads, protoplast fusion, viral infection, electroporation, “gene gun,” and DEAE- or calcium phosphate-mediated transfection.

Host cells transformed with expression vectors can be cultured under conditions suitable for the expression and recovery of the enzyme from the cell culture. The enzyme produced by a transformed cell can be secreted or contained intracellularly depending on the nucleotide sequence and/or the expression vector used.

Purification of Thermostable Omega-Transaminases

A recombinantly produced thermostable omega-transaminase can be isolated from a host cell engineered to produce the enzyme. A purified thermostable omega-transaminase is separated from other components in the cell, such as proteins, carbohydrates, or lipids, using methods well-known in the art. Such methods include, but are not limited to, size exclusion chromatography, ammonium sulfate fractionation, ion exchange chromatography, affinity chromatography, and preparative gel electrophoresis. A preparation of purified thermostable omega-transaminase is at least 80% pure; preferably, the preparations are 90%, 95%, or 99% pure. Purity of the preparations can be assessed by any means known in the art, such as SDS-polyacrylamide gel electrophoresis.

Chemical Synthesis of Thermostable Omega-Transaminases

Thermostable omega-transaminases of the invention can be synthesized, for example, using solid-phase techniques. See, e.g., Merrifield, J. Am. Chem. Soc. 85, 2149–54, 1963; Roberge et al., Science 269, 202–04, 1995. Protein synthesis can be performed using manual techniques or by automation. Automated synthesis can be achieved, for example, using Applied Biosystems 431A Peptide Synthesizer (Perkin Elmer). Optionally, fragments of a thermostable omega-transferase can be synthesized separately and combined using chemical methods to produce a full-length molecule.

Methods of Preparing Chiral Molecules

The invention provides methods of using thermostable omega-transaminases to prepare chiral molecules, either by enantiomerically enriching a mixture of chiral amines or by stereoselectively synthesizing one of a pair of chiral amines.

The enzymatic equilibrium reaction in which omega-transaminases participate is illustrated below:

in which each of R¹ and R², when taken independently, is an alkyl or aryl group which is unsubstituted or substituted with one or more enzymatically non-inhibiting groups and R¹ is different from R² in structure or chirality, or R¹ and R², taken together, are a hydrocarbon chain of 4 or more carbon atoms containing a center of chirality.

Amino Acceptors

An “amino acceptor” is a carbonyl compound which accepts an amino group from a donor amine. Amino acceptors include ketocarboxylic acids and alkanones. Typical ketocarboxylic acids are α-keto carboxylic acids such as glyoxalic acid, pyruvic acid, oxalacetic acid, and the like, as well as salts of these acids. Amino acceptors also include or substances which are converted to an amino acceptor by other enzymes or whole cell processes, such as fumaric acid (which can be converted to oxalacetic acid), glucose (which can be converted to pyruvate), lactate, maleic acid, etc.

Amino Donors

An “amino donor” is an amino compound which donates an amino group to the amino acceptor, thereby becoming a carbonyl species. Typical amino donors include the nonchiral amino acid glycine and chiral amino acids having the S-configuration such as L-alanine or L-aspartic acid; amino donors, however, need not be amino acids. For example, amines such as S-2-aminobutane, propyl amine, benzyl amine, etc. also can be used as amino donors.

Chiral Amines

Chiral amines have the formula:

in which each of R¹ and R² are as defined above. The compounds of Formulas IA and IB are enantiomers (or diastereomers if either R¹ or R² contains a second chiral center) and are chiral because R¹ is different in structure or chirality from R².

In chiral amines of Formulas IA and IB, the amino group is a primary amine and is bound to a secondary carbon atom, i.e., a carbon atom carrying one hydrogen atom and two substituents which are other than hydrogen (R¹ and R²). In addition, while R¹ and R² are selected from the same type of structures, these groups must render the molecule chiral; e.g., R¹ will be different from R² in structure or chirality or R¹ and R² when taken together are a chiral group. Generally, when taken independently, R¹ and R² will be alkyl, aralkyl, or aryl groups, preferably a straight or branched alkyl group of from 1 to 6 carbon atoms, a straight or branched phenyl-alkyl group of from 7 to 12 carbon atoms, or a phenyl or naphthyl group. Examples include methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, phenyl, benzyl, phenethyl, 2-phenylpropyl, etc.

Each R¹ and R² group optionally can be substituted with one or more groups, provided the groups do not significantly affect or compete with the action of the omega-transaminase. This can be readily determined by a simple inhibition assay. If inhibition is detected, it often can be minimized by conducting the reaction at lower concentrations of that reactant. Typical substituents without limitation include halo such as chloro, fluoro, bromo and iodo, hydroxy, lower alkyl, lower alkoxy, lower alkylthio, cycloalkyl, carbamoyl, mono- and di-(lower alkyl) substituted carbamoyl, trifluoromethyl, phenyl, nitro, amino, mono- and di-(lower alkyl) substituted amino, alkylsulfonyl, arylsulfonyl, alkylcarboxamido, arylcarboxamido, etc. Typical groups when R¹ and R² are taken together are 2-methylbutane-1,4-diyl, pentane-1,4-diyl, hexane-1,4-diyl, hexane-1,5-diyl, and 2-methylpentane-1,5-diyl.

Typical amines for which the present process is suitable include without limitation 2-aminobutane, 2-amino-1butanol, 1-amino-1-phenylethane, 1-amino-1-(2-methoxy-5-fluorophenyl)ethane, 1-amino-1-phenylpropane, 1-amino-1-(4hydroxyphenyl)propane, 1-amino-1-(4-bromophenyl)propane, 1-amino-1-(4-nitrophenyl)propane, 1-phenyl-2-aminopro-pane, 1-(3-trifluoromethylphenyl)-2-aminopropane, 2-aminopropanol, 1-amino-1-phenylbutane, 1-phenyl-2-aminobutane, 1-(2,5-di-methoxy-4-methylphenyl)-2-aminobutane, 1-phenyl-3-aminobutane, 1-(4-hydroxyphenyl)-3-aminobutane, 1-amino-2-methylcyclopentane, 1-amino-3-methylcyclopentane, 1-amino-2-methylcyclohexane, 1-amino-1-(2-naphthyl)ethane, 3-methylcyclopentylamine, 2-methylcyclopentylamine, 2-ethylcyclopentylamine, 2-methylcyclohexylamine, and 3-methylcyclohexylamine, 1-aminotetralin, 2-aminotetralin, 2-amino-5-methoxytetralin, and 1-aminoindan.

Thermostable Omega-Transaminases

Any of the thermostable omega-transaminases described above can be used to produce chiral compounds. A thermostable omega-transaminase can be used in free form (e.g., as a purified enzyme or in a cell-free extract). The enzyme optionally can be immobilized on a suitable support or matrix, such as cross-linked dextran or agarose, silica, polyamide, or cellulose. The enzyme also can be encapsulated in polyacrylamide, alginates, fibers, or the like. Methods for such immobilization are described in the literature (see, for example, Methods in Enzymology 44, 1976). The latter embodiment is particularly useful because, for example, once the immobilized enzyme is prepared one can simply feed the amino acceptor and a mixture of the chiral amines over the immobilized enzyme in order to effect the desired enrichment, and then remove the formed ketone. See U.S. Pat. Nos. 4,950,606; 5,300,437; and 5,346,828.

Enantiomeric Enrichment of a Chiral Amine

In some embodiments, thermostable omega-transaminases of the invention are used in methods for enantiomeric enrichment of a mixture of chiral amines. “Enantiomeric enrichment” is an increase in the amount of one chiral form compared to the other. This enrichment can involve (i) a decrease in the amount of one chiral form compared with the other, (ii) an increase in the amount of one chiral form compared with the other, or (iii) a decrease in the amount of one chiral form and an increase in the amount of the other chiral form. Enantiomeric enrichment typically is expressed as “enantiomer excess,” or “ee,” according to the following expression:

${cc} = {\frac{E^{1} - E^{2}}{E^{1} + E^{2}} \times 100}$ in which E¹ is the amount of the first chiral form of the amine and E² is the amount of the second chiral form of the same amine. For example, if the initial ratio of the two chiral forms is 50:50 and an enantiomeric enrichment sufficient to produce a final ratio of 50:30 is achieved, the ee with respect to the first chiral form is 25%, whereas if the final ratio is 70:30, the ee with respect to the first chiral form is 40%. Typically with methods of the present invention, an ee of 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99% or greater can be achieved. Enantiomeric enrichment can be determined by any means known in the art. For example, the ee of a given product can be determined by reaction with (−) α-(tri-fluoromethylphenyl)methoxyacetyl chloride (Gal, J. Pharm. Sci. 66, 169, 1977; Mosher et al., J. Org. Chem. 34, 25430, 1969) followed by capillary gas chromatography of the derivatized product on a Chrompack fused silica column.

Broadly, methods of enantiomeric enrichment involve subjecting a mixture of chiral amines to the action of a thermostable omega-transaminase of the invention in the presence of an amino acceptor.

in which R¹ and R² are as defined above and, in Formula III, either R³ is R¹ while R⁴ is R² or R³ is R² while R⁴ is R¹.

In general, the enzymatic process operates on only one chiral form, or operates on one chiral form to a far greater extent than the other. Because the reaction is an equilibrium, either the forward or reverse reactions can be favored by adding additional starting materials or removing reaction products. Thus, in an enantiomeric enrichment reaction, additional quantities of the amino acceptor can be added (up to saturation) and/or the ketone formed can be continuously removed from the reaction mixture. Conversely when one stereoselectively synthesizes one chiral form of an amine, as described below, additional ketone can be added (up to saturation) and/or the amine formed can be removed.

When the undesired chiral form of the amine is converted to the ketone and the desired chiral form is not, the latter can be readily isolated by conventional techniques. A partial separation can be effected by acidification, extraction with a hydrocarbon such as heptane to remove the ketone, rendering the aqueous phase basic, and re-extraction with a hydrocarbon such as heptane. When, on the other hand, both chiral forms of the amine are desired, the form which is converted to the ketone can be removed from the reaction mixture (or from the aqueous phase in a two phase mixture) and independently subjected to the action of an omega-transaminase in the presence of a amino donor to generate the same chiral form which was initially converted to the ketone.

Stereoselective Synthesis of a Chiral Form

Thermostable omega-transaminases of the invention also can be used in methods of stereoselective synthesis, i.e., to preferentially synthesize one chiral form of an amine of formula IA or IB in an amount substantially greater than the other. These methods typically involve subjecting a ketone of the formula:

in which R¹ and R² are as defined above to the action of a thermostable omega-transaminase of the invention in the presence of an amino donor until a substantial amount of one of the chiral amines is formed. “Substantially greater” as used herein refers to a percentage of the combined amount of both chiral forms of at least about 51% (e.g., at least 51, 55, 60, 65, 70 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99%).

All patents, patent applications, and references cited in this disclosure are expressly incorporated herein by reference. The above disclosure generally describes the present invention. A more complete understanding can be obtained by reference to the following specific examples, which are provided for purposes of illustration only and are not intended to limit the scope of the invention.

EXAMPLE 1

Stereoselective Synthesis and Enantiomeric Enrichment Reactions

To assess activity of a thermostable omega-transaminase in a stereoselective synthesis reaction, 108.8 g potassium phosphate is added to water in a reactor. One liter of concentrated HCl is added, and then isopropylamine is added. The pH is adjusted to 7–7.4, and the volume is adjusted to 8 L (i.e., 100 mM potassium phosphate and 1.0 M isopropylamine). Pyridoxal-5-phosphate is added to a final concentration of 2 mM, followed by 1–5 g of spray-dried crude thermostable omega-transaminase. Preferably the reaction takes place in the absence or near absence of oxygen, so nitrogen flow is started to purge the reaction mixture of oxygen. The enzyme is mixed for one hour under a nitrogen atmosphere, pH adjusted to 7–7.2, and the reaction is started by adding ketone 7-methoxy-2-tetralone to a final concentration of 100 mM. Samples are taken at regular intervals. Stability and activity are measured by monitoring the formation of amine product for at least 10 hours. FIG. 1 shows the typical progress of a synthesis reaction using the enzyme CNB03-03.

To assess activity of a thermostable omega-transaminase in an enantiomeric enrichment reaction, racemic methylbenzylamine is used as the amine donor, and sodium pyruvate is used as the amine acceptor; the rest of the components are the same.

EXAMPLE 2

Identification of Thermostable Transaminases

“CHIR-9867” (SEQ ID NO:6) is a mutant, thermostable transaminase. Mutants of CHIR-9867 were generated using error-prone PCR (Leung et al., J. Methods Cell. Mol. Biol. 1, 11–15, 1989) and screened for their activity at 60–68° C. compared to CHIR-9867 on a plate screen for the synthesis of S-7-Methoxy-2-aminotetralin (S7MAT). The plate screen is performed by subjecting the mutants to a reaction whereby an amine is converted into a ketone that becomes colored in the presence of oxygen. The rate at which the color is formed is used as an indication of the activity of the enzyme.

Thirty-eight potential candidates were identified from a primary plate screen. All identified mutants were further screened on plates, and thirteen mutants were identified as potential improved mutants. These 38 mutants appeared to have a better rate than the wild-type enzyme. These thirteen mutants were screened on 3 mL scale (reaction volume was 3 mL) at ≈65° C. The three best mutants were identified and named CNB03-01, CNB03-02, CNB03-03. These three identified mutants were further screened on 40 mL scale under a nitrogen atmosphere at different temperatures and enzyme concentrations to select the mutant with the best reaction rate. The reaction rate of CHIR-9867 was used as a control. The results obtained from the 40 mL scale screening are shown in Table 3.

TABLE 3 Initial Quantity Rate Enzyme # (g/L) Reaction Conditions (mM/hr) CNB 03-01 15 160 mM ketone, 650 68 mM donor amine, 60–62° C. CNB 03-02 15 160 mM ketone, 58 650 mM donor amine, 60–62° C. CNB 03-03 15 160 mM ketone, 71 650 mM donor amine, 60–62° C. CHIR-9867 15 160 mM ketone, 28 650 mM donor amine, 60–62° C.

All mutants showed high selectivity and the amine was synthesized with an ee>99%. Chiral purity was measured using HPLC with a chiral column.

CNB 03-01, CNB 03-02, and CNB 03-03 were sequenced. Their amino acid sequences are shown in SEQ ID NOS:8, 10, and 12, respectively. As expected, all of these mutant transaminases were altered in amino acid sequence compared to the amino acid sequence of CHIR-9867 (SEQ ID NO:6) from which they were generated. The changes in amino acid sequences between CNB 03-01, CNB 03-02, CNB 03-03, and CHIR-9867 are shown in Table 2.

Summary of HPLC analytical methods used for chemical and chiral analysis HPLC Method for Chemical Analysis Column: Waters, Nova-Pak Phenyl RCM 0.8 cm × 10 cm × 4 μm Mobile Phase: 25/75 (v/v-premixed) isopropanol/water containing 0.15% H₃PO₄ and 10 mM sodium octanesulfonate Flow Rate: 1.5 ml/min Injection Volume: 10 μL Temperature: Ambient Run Time: 20 min Detection: UV @ 254 nm Typical Retention times: ketone, 9–10 min; amine 13–14 min HPLC Method for Chiral Analysis Column: Diacel Crownpak CR (+) 15 cm × 0.40 cm × 5 μm Mobile Phase: 15/85 (v/v-premixed) methanol/ 1% HClO₄ in water. Flow Rate: 1.25 ml/min Injection Volume: 15 μL Temperature: 40° C. Run Time: 50 min Detection: UV @ 220 nm Typical Retention Times: R-amine 20–21 minutes; S-amine, 24–25 min.

EXAMPLE 4

Generation of a New Enzymes, CNB04-01, with Extended Thermostability

“CNB03-03” (SEQ ID NO:10) is a mutant, thermostable transaminase. Mutants of CNB03-03 were generated using error-prone PCR (Leung et al., J. Methods Cell. Mol. Biol. 1, 11–15, 1989) and screened for their activity after more than 24 hours at 60–68° C. compared to CNB03-03 on a plate screen for the synthesis of S-7-Methoxy-2-aminotetralin (S7MAT). The plate screen was performed by subjecting the mutants to a reaction whereby an amine is converted into a ketone that becomes colored in the presence of oxygen. The rate at which the color is formed was used as an indication of the activity of the enzyme.

Thermostability of CNB04-01 was tested as follows: enzyme was kept at 50° C. in a solution of 100 mM calcium chloride, 100 mM sodium acetate, 1 mM pyridoxal-5-phosphate at pH 8.5 (0.1 gram enzyme dissolved in 10 ml of solution). Periodically, 1 ml samples were withdrawn and added to 20 ml of reaction mixture pH 8.5 and incubated at 50° C. The change on (S)-aminotransferase activity as a function of time was measured. Results showed that the activity of the enzyme remained relatively stable for over a period of 150 hours with the initial rate at time zero of ˜36 mM/hour and an initial rate of ˜25 mM/hour after 150 hours.

EXAMPLE 5

Initial Rates of Activity of CNB05-01 (SEQ ID NO:16) and CNB05-02 (SEQ ID NO:18)

The initial rates of activity of the thermostable transaminases CNB05-01 (SEQ ID NO:16) and CNB05-02 (SEQ ID NO:18) were tested under the following reaction conditions: V=50 ml [2 mM PLP, 0.5 IPA, 200 mM sodium acetate], Enzyme: 5 g/l; (pH)0=7, ketone=130 mM, 55° C.

The results are shown in FIG. 2. From FIG. 2 it is clear that the initial rate of CNB05-01 and CNB05-02 is greater than CNB03-03. The initial rate of CNB05-01 was better than all the enzymes tested and was 0.39 mM/minute under the conditions tested (CNB03-03 had an initial rate of 0.21 mM/minutes).

EXAMPLE 6

Comparison of Enzymatic Activity of CNB05-01 (SEQ ID NO:16) and CNB03-03 (SEQ ID NO:12)

Enzymatic activity of the thermostable transaminases CNB05-01 (SEQ ID NO:16) and CNB03-03 (SEQ ID NO:12) were tested under the following conditions: 55° C., pH 7 (0–5 hr), 100 ml/min N2, 0.75 M IPA, 2 mM PLP, 200 mM sodium acetate, 2 ml ketone (130 mM), V=100 ml, nitrogen flow surface 100 ml/min.

The results are shown in FIGS. 3A and 3B. FIG. 3A demonstrates reproducibility of the results described in this example with the initial rate of CNB05-01 at 0.41 mM/minute and CNB03-03 at 0.25 mM/minute. FIG. 3B shows the results obtained during the course of the reaction with CNB05-01 achieving a higher conversion in a shorter period of time by virtue of having a higher initial rate. 

1. An isolated thermostable omega transaminase which comprises the amino acid sequence SEQ ID NO:16.
 2. An isolated polynucleotide which encodes the amino acid sequence SEQ ID NO:16.
 3. The isolated polynucleotide of claim 2 which comprises the nucleotide sequence SEQ ID NO:15.
 4. The isolated polynucleotide of claim 2 which is an expression construct.
 5. A method for enantiomeric enrichment of a mixture of two enantiomeric chiral amines comprising contacting: (a) a mixture comprising a first chiral amine and a second chiral amine which is an enantiomer of the first chiral amine, wherein the first chiral amine has the structural formula:

and wherein the second chiral amine has the structural formula:

 wherein each of R¹ and R², when taken independently, is alkyl, arylalkyl, or aryl and may be unsubstituted or substituted with one or more enzymatically non-inhibiting groups, and wherein R¹ and R² render the molecule chiral; (b) a thermostable omega transaminase which comprises the amino acid sequence SEQ ID NO:16; and (c) an amino acceptor for a time sufficient to convert the first chiral amine to a ketone such that the amount of the second chiral amine in the mixture is at least about 90% relative to the amount of the first chiral amine.
 6. The method of claim 5 wherein the contact is maintained at least until the amount of the second chiral amine in the mixture is at least about 99% relative to the amount of the first chiral amine.
 7. The method of claim 5 wherein the one or more enzymatically non-inhibiting groups are selected from the group consisting of halogen, hydroxy, lower alkyl, lower alkoxy, lower alkylthio, cycloalkyl, carbamoyl, mono-(lower alkyl)-substituted carbamoyl, di-(lower alkyl)-substituted carbamoyl, trifluoromethyl, phenyl, nitro, amino, mono-(lower alkyl) substituted amino, di-(lower alkyl) substituted amino, alkylsulfonyl, arylsulfonyl, alkylcarboxamido, arylcarboxamido, 2-methylbutane-1,4-diyl, pentane-1,4-diyl, hexane-1,4-diyl, hexane-1,5-diyl, and 2-methylpentane-1,5-diyl.
 8. The method of claim 5 wherein the amino acceptor is a ketocarboxylic acid.
 9. The method of claim 8 wherein the ketocarboxylic acid is selected from the group consisting of glyoxalic acid, pyruvic acid, oxalacetic acid, and salts thereof.
 10. The method of claim 5 wherein the amino acceptor is selected from the group consisting of oxalacetic acid and pyruvic acid.
 11. A method for stereoselective synthesis of one of two chiral forms of an amine comprising contacting: (a) a ketone having the structural formula:

wherein each of R¹ and R², when taken independently, is alkyl, arylalkyl, or aryl and may be unsubstituted or substituted with one or more enzymatically non-inhibiting groups; (b) a thermostable omega transaminase which comprises the amino acid sequence SEQ ID NO:16; and (c) an amino donor for a time sufficient to form a substantially greater amount of a first chiral form of the amine than the amount of a second chiral form of the amine.
 12. The method of claim 11 wherein the one or more enzymatically non-inhibiting groups are selected from the group consisting of halogen, hydroxy, lower alkyl, lower alkoxy, lower alkylthio, cycloalkyl, carbamoyl, mono-(lower alkyl)-substituted carbamoyl, di-(lower alkyl)-substituted carbamoyl, trifluoromethyl, phenyl, nitro, amino, mono-(lower alkyl) substituted amino, di-(lower alkyl) substituted amino, alkylsulfonyl, arylsulfonyl, alkylcarboxamido, arylcarboxamido, 2-methylbutane-1,4-diyl, pentane-1,4-diyl, hexane-1,4-diyl, hexane-1,5-diyl, and 2-methylpentane-1,5-diyl.
 13. The method of claim 11 wherein the amino donor is selected from the group consisting of glycine, L-alanine, L-aspartic acid, S-2-aminobutane, propyl amine, and benzyl amine. 